According to the American Cancer Society, approximately 180,000 American women were diagnosed with breast cancer in 1993. Approximately 45,400 died from the disease in the United States, breast cancer continues to be the most common of the nonpreventable cancers diagnosed among women. Much of the urgency in improving early diagnosis of breast cancer stems from the tragic and steady rise in the incidence of same . . . 1 in 16 women in 1962, to 1 in 9 women in 1993. In fact, the incidences of breast cancer have been increasing steadily in the United States since formal tracking of cases through registries began in 1930. There has been no appreciable change in the death rate during the same period.
It has long been known that early detection of malignant tumors increases the chance of surival. In fact, the results of a 1976 National Institute of Heath Survey indicate a dramatic increase in survival (from 56% to 85%) as a result of early detection.
The technique that is used most often to screen for breast tumors is mammography. However, mammography exposes the patient to x-rays. This involves some hazard even though the radiation dosages are relatively small because multiple exposures of each breast are required in order to cover all quadrants of the breasts. Also, the procedure can be somewhat time consuming because the technician taking the mammogram is required to position the apparatus prior to taking each picture and then take refuge behind a leaded window while the x-ray tube is operative to avoid undo exposure to the radiation. Most importantly, mammography is disadvantaged because when adjusting the apparatus to the patient in order to obtain enough compression of the breast to ensure adequate x-ray penetration, the working end of the apparatus is often positioned so that the x-rays fail to properly illuminate the upper half of the breast and the axillary gland area above the upper outer quadrant of the breast where most tumors originate. This is particularly so for individuals with small breasts. Indeed, studies have shown that 80% of all breast tumors occur in the upper half of the breast and fully 69% arise in the upper outer quadrant of the breast and the axillary gland area. Resultantly, mammography misses many tumors.
Another technique for measuring tumors is thermography. Thermography relies on the fact that tumors tend to have higher temperatures than normal tissue due to the higher metabolic activity and vascularity of tumors. Therefore, the tumors tend to appear as hot spots in a thermogram of the breast. Thermography has definite advantages over mammography because it is non-invasive and non-hazardous both to the patients and to the personnel taking the thermograms.
The most common type of thermography is infrared thermography. Diagnostic techniques using electromagnetic emission in the infrared region of the spectrum have been available for many years and have proved useful in measuring surface temperature distributions in the body. However, body tissue rapidly absorbs electromagnetic energy at the infrared frequencies. Since the heat associated with a subcutaneous tumor is transferred by radiation as well as convection and conduction, the thermal pattern seen at the slain surface due to such a tumor can be altered significantly. In fact, in some cases, a relatively deep tumor may not appear at all in an infrared thermogram of the affected area. Thus, infrared thermography is essentially limited to surface measurements which can vary greatly in response to external factors such as physical activity, menstrual al cycle, substance intake, etc.
More recently there has been developed thermography systems for locating tumors in the body using microwave radiometry. These systems, which operate at the lower microwave frequencies, provide improved transmission characteristics in tissue and, therefore, allow detection at greater depths in tissue. Two such systems for detecting cancerous tumors are described in my U.S. Pat. Nos. 4,346,716 and 4,774,961. Both of these systems screen for tumors by detecting radiometrically the increased energy emitted in the microwave band by the relatively hot cancerous tumors. The system described in the former patent utilizes a single relatively small detection antenna. Therefore, it is impractical for use in screening for breast tumors because it takes too long to probe all quadrants of the breast. The microwave detection apparatus described in the latter patent avoids this problem to some extent by employing a detection antenna array composed of a relatively large number (i.e., 6-12) of individual antennas which are switched, in turn, to a single radiometer. This allows the apparatus to image the entire breast, or at least a large area thereof, with each positioning of the apparatus relative to the breast.
When screening for breast tumors using the microwave radiometry system in my '961 patent, the usual procedure is to compare the temperatures at common locations on the two breasts of the patient to determine if there is a temperature difference. In other words, absent tumors, there is a surprising correspondence of temperatures at corresponding locations on opposite sides of a given individual, i.e., a temperature differential within about 0.2.degree. C. Consequently, if a larger temperature differential does exist at corresponding locations on the two breasts, this is an indication that an abnormality may be present in the breast with the higher temperature reading. The usual practice, then, is to take temperature readings at various locations on one entire breast and then reposition the apparatus to take similar measurements at corresponding locations on the other entire breast. These readings are fed to a controller with data processing capability which compares them in order to produce a thermogram or other visual display showing the temperature differentials at the corresponding locations on the two breasts. Usually also, the temperature-indicating signals are processed using various known averaging, enhancement and target recognition techniques to increase the probability that a tumor-indicating hot spot will be recognized in the display.
When scanning for breast tumors using a multiple-antenna array according to the above procedure, it must be taken into consideration that an individual's breasts are handed. In other words, when facing an individual, the axillary gland area of the individual's right breast is to the left of the observer, while the axillary gland area of the individual's left breast is to the right of the observer. Therefore, if an antenna array is positioned against that individual's right breast, the antenna in the upper left corner of the array will be closest to the gland area of that breast. On the other hand, if the same array is pressed against that individual's left breast, the antenna in the upper right corner of the array will be closest to the gland area of that breast. This means that when temperature measurements are being taken of both breasts, the temperature information from the various antennas in the array being fed to the system controller must be switched so that proper comparisons are made of common points on the two breasts.
The multiple antenna arrangement described in my '961 patent is disadvantaged in that it requires breast compression so as to reduce the tissue thickness being examined in order to obtain accurate tissue temperature measurements of the breasts. In one embodiment of that patented system, a single multiple antenna array is held against the breast in order to compress the breast. In a second embodiment of that system, the breast is compressed between a pair of opposed multiple antenna arrays. In both cases, it has proven difficult to obtain the required intimate contacts between all of the antennas in the antenna array(s) and the surface of the breast for all areas of the breasts, with the result that sore& tumors may go undetected.
Also, when screening for tumors in relatively small breasts, in order to adequately compress the breast with the antenna array, the upper array has to be positioned so that, like the prior mammography systems described above, it may fail to detect tumors in the upper half of the breast and the axillary gland area above the breast.
Another disadvantage of having to compress the breast in order to screen for tumors is that the very act of compression upsets the blood circulation in the breast tissue and causes temperature changes therein. Therefore, after the antenna array(s) has been pressed against the breast, it is necessary to wait a couple of minutes to allow the breast temperature to stabilize before taking temperature measurements. This obviously increases the overall breast examination time. Also, the maintenance of the breast under compression causes discomfort to some patients and makes them more reluctant to undergo the breast screening procedure.
Further, in the prior multiple antenna detection systems described above, the individual antennas are arrayed in a stack or in offset courses like bricks in a wall. Resultantly, the array has a relatively large footprint or there may be gaps between the antenna patterns of the array which may allow some tumor-indicating hot spots to be missed during the breast examination.
Finally, the use of a multiple antenna array time-shared with a single radiometer introduces switching artifacts into the signals from the radiometer which can degrade the temperature readings obtained by the prior system.
For all of the above reasons, microwave radiometry is not as widely used to screen for breast tumors as might be expected considering the advantages which it offers in terms of detection penetration depth, safety and efficiency.